1. Field of the Invention
The present invention relates to a liquid-drop ejection device for a drop-on-demand printer. More specifically, the present invention relates to a method for more effectively and accurately forming electrodes on the ejection device, and the ejection device produced by the method of the present invention.
2. Description of Related Art
A piezoelectric liquid-drop ejection device or ejector incorporated into a printer head to form a piezoelectric drop-on-demand ink jet printer has recently been proposed. The above ejection device is constructed such that the capacity of an ink chamber is varied depending on a variation in the orientation of a piezoelectric actuator, thereby ejecting ink from the ink chamber upon a reduction in the capacity of the ink chamber and drawing ink from an ink supply into the ink chamber upon an increase in the capacity of the ink chamber. Desired characters and images can be formed by providing a plurality of ejectors adjacent to one another and controllably ejecting ink from the plurality of ejectors.
This type of liquid-drop ejector has been disclosed in U.S. Pat. No. 5,028,936, 5,003,679 and 4,992,808, all to Bartley et al. for example. The conventional liquid-drop ejector will be described with reference to prior art FIGS. 7-9 of the present application. FIG. 8 shows a cross-sectional view of a portion of an array 1 of ejectors. A plurality of parallel ink channels 4 are spaced at given intervals from each other along the transverse direction of the ejector array 1. The channels 4 are defined by joining a piezoelectric ceramic plate 2 having a plurality of vertically extending side walls 3 to a cover 6. The side walls 3 are subjected to polarization processing in the direction indicated by the arrow D. Each of the ink channels 4 is shaped in the form of a long and narrow rectangular prism. Each of the side walls 3 extends along the entire length of each ink channel 4 and can be moved in a direction perpendicular to the long axis of each ink channel 4 to vary the pressure in the ink in each ink channel 4. Drive electrodes 5 apply driving electric fields to the side walls 3 and are formed only on the upper half (or alternately only on the lower half) of the side walls 3. The surfaces of the drive electrodes 5 are processed to be electrically insulated from the ink in the ink channels 4. Each ejector 7 of the ejector array 1 comprises an ink channel 4, a corresponding nozzle (not shown) which communicates with one end of the ink channel 4, an ink supply (not shown) which communicates with the other end of the ink channel 4, and the piezoelectrically deformable side walls 3 which define the ink channel 4.
Next, a drive circuit of the liquid-drop ejector is described below with reference to FIG. 9, which shows a cross-sectional view of the array 1. In the array 1, ink channels 4A through 4C are respectively formed by a cover 6, a piezoelectric ceramic plate 2 and side walls 3A through 3D of the piezoelectric ceramic plate 2. Drive electrodes 5A through 5H are formed on the corresponding upper half of the side walls 3A through 3D. The drive electrodes 5A through 5H are electrically connected to a CPU 11. The CPU 11 selects any one or more of the ejectors 7A through 7C to be driven in accordance with printing data, and controls the drive electrodes 5A-5H to drive the ejection devices 7A through 7C.
When the CPU 11 selects the ejection device 7B in response to predetermined printing data, for example, it applies driving electric fields between the drive electrodes 5C and 5D and between the drive electrodes 5E and 5F. At this time, the direction of application of the driving electric fields 10 meets at a right angle to the direction D of polarization. Therefore, the drive electrodes 5C, 5D, 5E and 5F cause the upper (or lower) half of the side walls 3B and 3C to deform under a piezoelectric thickness sliding effect. Accordingly, the side walls 3B and 3C are deformed to form doglegs angled away from the ink channel 4B, thereby increasing the capacity of the ink channel 4B. Accordingly, additional ink is drawn into the ink chamber 4B from the ink supply. When the CPU 11 is deactivated to remove the driving electric fields 10 from the adjacent drive electrodes 5C-5F, the side walls 3B and 3C return to their original positions. The pressure in the ink within the ink chamber 4B increases as the piezoelectric deformation of the side walls 3B and 3C first increases then decreases the capacity of the ink chamber 4B. As the ink chamber 4B is now overfilled with ink, droplets of ink are ejected from the nozzles connected to ink chamber 4B.
FIG. 7 shows a method for forming the conventional drive electrodes 5 employed in the liquid-drop ejectors. This conventional liquid-drop ejector forming method is described below. A piezoelectric ceramic plate 2 (or the like) is formed of a lead zirconate titanate (PZT) ceramic having a strong dielectric characteristic and is subjected to polarization processing along the direction indicated by the arrow D. The plate 2 is first provided with ink channels 4 by cutting a plurality of parallel grooves with a rotating diamond cutting disc or the like. Then, the drive electrodes 5 are formed on the side surfaces of the side walls 3 by a vapor deposition process. At this time, the piezoelectric ceramic plate 2 is inclined with respect to a target or a vapor deposition source. As a result, the drive electrodes 5 can be formed on the desired regions of the surfaces of the side walls 3 through the aperture or opening of the ink channels 4 formed between adjacent side walls 3, due to the shadow effects of the adjacent side walls 3. The electrode portions 51 formed on the end surfaces of the side walls 3 are removed by lapping or the like.
In the conventional liquid-drop ejector forming method, however, the drive electrodes are formed on only one side of each side wall 3 at a time. It is therefore necessary to execute two vapor deposition steps in order to form the drive electrodes 5 on both sides of each side wall 3. Strictly speaking, it is also difficult to form the drive electrodes 5 only on the desired portions (i.e., the upper or lower half) of the side walls.